The present invention relates to a helical scan type of video tape recorder (abbreviated in the following to VTR), and more particularly to an improved method and apparatus for segment recording of video signals, i.e. recording of each field of a video signal as a plurality of segments, which are recorded on respective diagonal tracks on a magnetic tape.
A helical scan type of VTR employs a rotating cylinder, generally referred to as the head drum, having at least one pair of electromagnetic recording/playback heads mounted on the drum periphery, spaced apart by 180.degree.. The most generally utilized recording systems are the VHS and the Beta systems. FIG. 1 is a simple plan view to illustrate the positional relationships between the magnetic tape and the head drum of a helical scan VTR. FIGS. 2(a) and 2(b) respectively are diagrams to illustrate the recording format and the manner of scanning the image which is recorded and played back, for the case of a conventional recording method in which each field of the video signal is recorded to occupy a single recording track on the magnetic tape. In FIG. 1, numeral 1 denotes a magnetic tape, and 2 and 3 denote respective electromagnetic recording/playback heads (referred to in the following simply as heads), while numeral 4 denotes a rotary drum having heads 2 and 3 fixedly mounted at diametrically opposed positions on the periphery thereof. The direction of transport T of tape 1 is oriented diagonally with respect to the plane of rotation of heads 2 and 3 (rotating in the direction indicated as H), so that recording tracks produced by heads 2 and 3 are successively aligned diagonally along the tape 1. A successively recorded set of these tracks is shown in FIG. 2(a), indicated by numeral 5. Each of the tracks 5 is produced by rotation of one of the heads 2 or 3 through 180.degree.. Assuming a 2:1 interlace system, in which each frame of the video signal consists of two successive fields, the magnetic tape 1 will advance by a distance P shown in FIG. 2(a), during recording of each frame of the video signal, i.e. one field is recorded on each track. With such a VTR, designating as A and B the positions at which two horizontal scanning lines of a first field are recorded on tape 1 (at the beginning and end, respectively of one of tracks 5), and C and D the positions at which two corresponding lines of the succeeding field are recorded, then the positions at which these lines will appear on the displayed image following playback are as shown in FIG. 2(b).
With such a recording method, since each field of the video signal is recorded in one track on the tape, the speed of rotation of the head drum (in revolutions per second) must be identical to the frame repetition frequency (in Hz) if a 2:1 interlace ratio is employed.
In order to design a two-head helical scan VTR which is capable of recording wide-band video signals of a high-definition television system, in accordance with NTSC standards, it is possible to adopt either of the following methods:
(a) Dividing the input signal into a plurality of channels for recording purposes.
(b) Increasing the relative velocity between the magnetic tape and the heads, by increasing the speed of rotation of the head drum.
A video recording apparatus for wide-band video signals can be implemented by utilizing either of methods (a) or (b) above, or a combination of both methods. If method (b) is used, then it is essential to employ a segment recording technique. A 2-segment recording method will be briefly described here. In order to record a high-definition NTSC video signal having double the bandwidth of a conventional video signal, it is necessary to make the speed of rotation of the head drum twice the conventional speed of rotation (i.e. corresponding to twice the frame repetition frequency), and to double the transport speed of the magnetic tape. In this case the relationship between the recording format and the displayed image, for the case of a prior art 2-segment recording method, will be as shown in FIGS. 3(a) and 3(b). In FIG. 3(a), P' denotes the distance through which the tape 1 is transported during one field interval. Each field is divided into two equal segments, each of which is recorded to occupy one of tracks 5 on tape 1. A' and B' denote positions at which two horizontal scanning lines of a first segment of a field are recorded on tape 1 respectively at the beginning and end of one of tracks 5, and C' and D' the positions at which two corresponding lines of the other segment of that field are recorded. The positions at which these lines will appear on the displayed image following playback are indicated by the corresponding letters A' to D' in FIG. 3(b), in which SG1 denotes a first segment and SG2 a second segment.
With such a prior art 2-segment recording method, it is found that the following serious problems arise:
(1) If a non-standard playback operating mode is executed, such as slow-motion playback, still playback or rapid-motion playback (these being collectively referred to in the following as varied-speed modes, for simplicity of description), in which sets of tracks are successively skipped over by the heads, then the two segments SG1, SG2 of each field will become mutually intermingled in such a manner that a satisfactory image cannot be displayed.
(2) The high-frequency signal which is produced from each head during playback (referred to in the following as the RF, or radio-frequency, signal) is unstable in level during the times at which each head traverses a region of the magnetic tape which is close to an edge of the tape. As a result, the signal/noise ratio of the playback signal which is produced as output by the VTR will be unstable during playback of these portions of the tape. However with the prior art 2-segment recording method described above, parts of the displayed image corresponding to these playback portions from the tape edge will appear at the center of the display, and the resultant effects are therefore very conspicuous.
Problems (1) and (2) above will be described in more detail, referring first to FIGS. 4(a), 4(b) and 4(c). FIG. 4(a) illustrates the recording pattern produced by the prior art 2-segment recording method of FIG. 3. Numerals 6 and 6' respectively denote the paths which are traced out by the two heads 2 and 3 when the magnetic tape 1 is advanced at four times the standard speed during playback. A set of 11 successive tracks is respectively designated as F1 to F11, with the segment pairs of four successive fields (designated as the first to the fourth fields in FIG. 4(b)) being recorded in tracks F1 to F8. Thus as indicated in FIG. 4(b), during operation at normal tape transport speed, the contents of track F1 will appear in the upper half of the display and the contents of F2 in the lower half of the display during the first field. The display positions of the contents of tracks F3 to F8 during normal playback operation are similarly indicated in FIG. 4(b) for the second to the fourth fields. The hatched portions S1 to S8 in FIG. 4(b) indicate the positions at which the contents of the tape track portions S1 to S8 of fields F1 to F8 shown in FIG. 4(a) will appear on the display, during operation at four times the normal playback speed, i.e. as the heads 2 and 3 move along paths 6 and 6' respectively.
FIG. 4(c) illustrates scanning of a display produced by playback at four times normal speed corresponding to FIG. 4(b). As shown, portions of the upper half and lower half of the image to be displayed, each having a width which is equal to 1/8th of the total image height, are intermingled in an alternating manner. The displayed image will therefore be confused and unsatisfactory.
In the above example, high-speed playback at four times normal playback speed is utilized. However similar intermingling of portions of the image will occur in general when playback is performed at S times the normal speed, or when slow-speed playback is performed at 1/S times the normal speed, where S is an integer.
As will be clear from the above, the order in which the segments are played back will depend upon the tape transport speed. Thus if it is attempted to produce a correct displayed image which is free from the intermingling effect described above, by performing appropriate signal processing during playback operation, such processing will be extremely difficult to put into practice. This is because the processing must be varied in accordance with different tape transport speeds.
The second problem (2) mentioned above can be understood by referring to FIG. 5, which shows the form which the envelope of the playback RF signal may take. FIG. 5 shows envelope portions corresponding to two segments, with A', B', C' and D' corresponding to the identically designated playback timings of FIGS. 3(a) and (b). The reductions in envelope amplitude which occur at times B' and D' are due to the heads being in insufficiently close contact with the tape at these times, i.e. as the heads scan across positions near an edge of the tape. As a result, the output signal from the VTR will have a relatively low S/N ratio at time B' and relatively high S/N ratio at time C'. These times correspond to positions near the center of the display produced by playback from the VTR, so that a very conspicuous unevenness of picture quality is produced.
The above problems of the prior art have been described for the case of the 2-segment recording method. However such problems will become increasingly severe if the number of segments is increased. More specifically, in the case of problem (1) above, portions of a greater number of segments will become intermingled in the displayed image during playback. In the case of problem (2) above, a greater number of regions of display quality irregularity will appear on the displayed image.